President Donald Trump on Wednesday blamed China for targeting U.S. farmers in an effort to undermine trade negotiations with Beijing, and he urged critics of his escalating trade war to “be cool.”

“China is targeting our farmers, who they know I love & respect, as a way of getting me to continue allowing them to take advantage of the U.S.” Trump posted in one of a series of tweets on Wednesday. “They are being vicious in what will be their failed attempt. We were being nice – until now!”

The tweets come as Trump’s trade policy is increasingly under fire from Republicans on Capitol Hill, especially those representing farm states where China’s retaliatory tariffs are affecting crop prices. The Trump administration rolled out a $12 billion subsidy plan on Tuesday to help farmers, but the measures were widely criticized by Republicans.

“When you have people snipping at your heels during a negotiation, it will only take longer to make a deal, and the deal will never be as good as it could have been with unity,” Trump wrote Wednesday. “Negotiations are going really well, be cool. The end result will be worth it!”

There is also a domestic political component to Trump’s trade and tariff policies.

Trump political advisers are clearly worried about how tariffs are affecting farmers, key sources of votes throughout the Midwest. They could be decisive as Republicans face tough election battles in November to keep control of the House and Senate.

The president himself expressed a rare note of anxiety in a speech to veterans Tuesday in Kansas City, Mo., a key agricultural state.

“The farmers will be the biggest beneficiary” of his trade policies, Trump said. “Watch. We’re opening up markets. You watch what’s going to happen … Just be a little patient.”

Trump travels Thursday to two more agriculture states, Iowa and Illinois.